c++ - 表达式包含不匹配的括号

Question

我的程序给我一条错误消息:libc++abi.dylib: terminating with uncaught exception of type std::__1::regex_error: The expression contained mismatched ( and ) 我可以'不知道如何修复它。括号是什么？

#include <regex>
#include <iostream>
#include <string>
using namespace std;
using namespace regex_constants;

int main() {
  // here can be url, or hashtag or split string: 
  // www.example.com or #followback or currentratesoughttogodown and output must be example or followback or currentratesoughttogodown.
  string input = "www.example.com"; 
  smatch m;
  regex_match ( input, m, regex ("(?<=www.|\\#)(\\w+)(?=\\.)?") );

  return 0;
}

我用gcc编译:

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.1.0
Thread model: posix

Answer 1

C++11 uses ECMAScript's (ECMA-262) regex syntax, so it will not have look-behind (other flavors of regex that C++11 supports also doesn't have look-behind).

Lookahead即(?=)虽然支持

你或许可以试试 (?:www\\.|\\#)(\\w+)(?=\\.)

虽然捕获第 1 组。或者你可以看看使用 Boost.Regex或 Boost.Xpressive支持 lookbehind .